The application of combinatorial methodologies to the discovery of new materials such as polymers continues to receive considerable attention in academia and industry because it has the potential to increase greatly the rate of discovery over conventional discovery methods. For the discovery of new materials, such methodologies have great utility in the field of homogeneous catalysis, which often employs organometallic catalysts. U.S. Pat. No. 6,030,917, incorporated herein by reference, owned by the Assignee of the present application and entitled Combinatorial Synthesis and Analysis of Organometallic Compounds and Catalysts, issued February 2000 to Weinberg et al., discusses general combinatorial methods for preparing organometallic compounds such as catalysts. Organometallic compounds and catalysts may be generated by the reaction of metal precursors with ligands. PCT Application No. PCT/US00/00418, incorporated herein by reference, published July 2000 and owned by the Assignee of the present invention, discusses library formats for ligand arrays that may be used in the application of combinatorial methodologies. Typically, active catalysts are then generated by treatment of said organometallic compounds and catalysts with chemical components such as suitable activators and/or scavengers. Often, it is preferable to generate active catalysts in the presence of at least one of the monomers, and even more preferably all the monomers, to be polymerized, and under conditions where parameters such as monomer concentrations, reactant ratios, partial pressures of gases and temperature are carefully controlled. Without careful control over such experimental parameters the performance of the catalyst and the nature of the product may be adversely affected. For example, catalysts may decompose more readily in the absence of the monomer(s) to be polymerized. Generation of an active catalyst is generally preferably done under pressure and temperature equilibrated conditions, and is especially important and challenging for polymerization involving the use of gaseous monomers such as ethylene, propylene, vinyl chloride and isobutylene.
A typical primary screening workflow that utilizes combinatorial methodologies for the discovery of new catalysts such as homogeneous catalysts that may polymerize olefinic monomers involves the screening of large arrays of potential catalysts for their activity and ability to produce polymers or copolymers with desired polymer properties such as molecular weight distribution, comonomer content, sequence distribution, melting point, mechanical and rheological properties and the like. The most promising catalysts as judged by, for example, their activity and the molecular weight distribution of the polymers they produce may be screened later under more carefully controlled conditions to further probe or ascertain polymer properties, composition, or structure.
Often, there is a need for conducting primary screening experiments at elevated temperatures and pressures, and for carrying out post-reaction characterization of the products. Precise control of variables such as temperature, agitation, pressure, and timing and sequence of addition of reagents is often needed. Without careful control screening results may be compromised by uncertainties in the contributions from these experimental factors.
Conventional primary screening reactor designs do not typically allow for the addition of reagents such as scavengers and activators at a desired reaction temperature or in the presence of pressure-equilibrated gaseous monomers. For example, arrays of catalyst precursors are often activated at room temperature and are rapidly transferred to a heater where the reaction vials within the array gradually reach the desired screening temperature. The time required to reach the screening temperature introduces an ambiguity with respect to the temperature at which the polymerization actually occurs. The performance of catalysts is expected to be dependent on temperature. For example, some catalysts may be active at room temperature yet are less active or decompose at higher temperatures. These catalysts may appear as false positives in primary screening experiments using past methodologies involving a temperature ramp during the polymerization, and may lead to unnecessary follow-up studies. Properties of the polymer product produced are also expected to be strongly dependent on temperature. As a consequence, rating the capability of catalysts to produce certain products as a function of temperature may be difficult without the capability of activating catalysts at the reaction temperature and at set monomer pressures.
The inability to activate catalyst precursors under pressure equilibrated conditions in past primary screening processes may also lead to false negatives. For example, catalysts are typically activated at room temperature either in the presence of a small amount of solubilized monomer or in the absence of monomer. These small amounts of monomer may not be able to stabilize very active catalysts to the degree that would be achieved if all the desired monomer were present. Without this stabilization by monomer, these catalysts may degrade.
In another conventional primary screening process, a high pressure reactor containing arrays of catalyst precursors is pre-pressurized with the target monomer mixture in order to allow the monomer to dissolve into the wells of the array. The reaction chamber is depressurized to allow the library to be removed so that, for example, activators may be added, resulting in the outgassing of gaseous monomers from the solutions in the wells of the array. As a result, the gaseous monomers are no longer present at the same concentration that they were when under pressure, leading to uncertainties about the amount and ratio of gasses dissolved. For polymerization reactions involving more than one olefinic monomer, this may mean that the monomer ratios are different at the time of activation. This event creates a less than ideal situation because the monomer ratio and concentration may be critical to catalyst performance as well as polymer composition, property and structure. For example, in a vinyl chloride-ethylene mixture, outgassing of the more volatile ethylene may lead to undesirably high vinyl chloride concentrations, possibly causing catalyst degradation.
Another drawback to traditional primary screening reactors is their limited stirring capabilities that results from the use of conventional rotary stirring. Reactors currently used in primary screening are typically placed on top of conventional magnetic stir plates, which are used to move stir bar elements located in individual wells. However, the rates of stirring may depend strongly on the location of the wells relative to the center of the stir plate, which can have a dramatic affect on the ability of gaseous reagents to diffuse into reaction mixtures within the wells. This may lead to inhomogeneous diffusion rates for gaseous monomers such as ethylene from well to well within a library, resulting in ambiguities in data obtained for diffusion-limited reactions.
For the aforementioned reasons, it is desirable to activate the catalyst precursor libraries once pressure and temperature equilibration have been reached. Additionally, it is desirable to provide homogenous stirring to libraries while being screened in such high throughput reactors.